1. Field of the Invention
This invention relates to gas turbine engines and more specifically to apparatus for reducing the leakage of working medium gases between the rotor and stator assemblies of a gas turbine engine.
1. Descripton of the Prior Art
Scientists and engineers within the gas turbine engine field have long recognized that substantial performance penalties are imposed upon engines by the leakage of working medium gases between the rotor and stator assemblies of an engine. They have also recognized that a substantial portion of the leakage is attributable to the radial clearance between the assemblies which is required to accommodate differential thermal expansion between the rotor and stator assemblies as the assemblies are exposed to diverse thermal environments.
At one particular interface between the rotor and stator assemblies, a turbine outer air seal surrounds and opposes the tips of the rotor blades. The outer air seal is in intimate contact with the working medium gases of the engine flow path and, accordingly, responds rapidly to variations in gas path temperature. The turbine case is located remotely from the gas path and responds much more slowly to changes in operating conditions. The outer air seal is, conventionally, segmented to prevent the buildup of mechanical stresses within the outer air seal. As the engine is accelerated during operation, the rotor assembly grows radially outward toward the outer air seal. A substantial initial clearance is provided between the seal and the blade tips to permit this radially outward growth free of destructive interference between the rotor assembly and the outer air seal. The minimum clearance between the blade tips and the outer air seal occurs during transient operating phases such as takeoff and acceleration. At equilibrium conditions, however, the clearance again increases and excessive leakage of working medium gases between the rotor and stator assemblies occurs.
The radial clearance between the rotor and stator assemblies at cruise conditions in modern engines is reduced through the incorporation of turbine case cooling systems. As the turbine case is cooled the outer air seal is forced radially inward toward the tips of the rotor blades to a position of more acceptable clearance. Although turbine case cooling holds considerable promise for improved performance, engines incorporating such systems have yet to obtain their predicted improvement.
Continuing efforts are underway to provide turbine case cooling systems which are capable of substantially improving the performance of a gas turbine engine by minimizing the clearance between the rotor assembly and the stator assembly.